Polyimide and a precursor thereof are used as a base film of a printed circuit board, a cover film for a large scale integrated semiconductor device or a printed circuit board, or the like since they have an excellent durability, a heat resistance, a flame retardant property, and mechanical and electrical properties,
For example, in a flexible circuit board, a polyimide film is utilized as a base film that is used together with copper (Cu) of a Copper Clad Laminate (hereinafter, CCL) with a conductive circuit pattern formed thereon. It is also utilized as a coverlay film (i.e., a cover film) for protecting the conductive circuit pattern and for enhancing a bending resistance. Such a coverlay film is processed in advance through punching before it is laminated on the surface of the CCL with a circuit formed thereon by using a thermal compression method. However, as the electronic devices are getting smaller and required to have multiple functions, a printed circuit board to be used therein becomes highly densified and the circuits become fine. Since the coverlay film is punched beforehand and then is aligned and layered with a circuit on the CCL by an operator, a precision alignment is a daunting task and indeed imposes limitations in meeting the demands for a fine circuit or a highly densified printed circuit board.
There were recent attempts to resolve these problems by adopting a photolithography process using a photosensitive resin composition. In this attempt, the photosensitive resin composition was thermally compressed on a circuit of the CCL, exposed to light according to a pattern, and then developed to give a patterned structure, by which a fine hole can be punched more precisely and thus it is possible to meet the demands for a fine circuit and the like.
In this regard, the majority of photosensitive resin compositions known to be available for such uses need to use a flammable organic solvent or a toxic solvent as a developing solution, but such solvents may cause problems in a working stability or the environment. Therefore, recently, a photosensitive resin composition developable with an alkaline aqueous solution continues to be demanded.
Also, in order for the photosensitive resin composition to be used in forming the coverlay film as described above and thus to effectively protect the circuit, a dry film made therefrom or its cured product needs to satisfy requirements for a variety of desired properties such as a solder heat resistance, a folding endurance (brittleness and a bending resistance), a chemical resistance, an electrically insulating property, a flame retardant property, and the like.
However, no photosensitive resin composition previously known in the art can be developed with an alkaline aqueous solution while satisfying the requirements for all the foregoing properties.
In a bid to deal with these problems, there was an attempt to develop a photosensitive resin composition based on polyimide. This is because the polyimide was used for protection of the circuit while satisfying a heat resistance, a bending resistance, or a insulating property. Despite such an attempt, however, a polyimide based photosensitive resin composition has following drawbacks when being used in forming a circuit protection film such as the coverlay film.
Because a photosensitive resin composition comprising polyimide itself is disadvantageous in forming a film, such photosensitive resin composition needs to comprise a precursor of polyimide, for example a polyamic acid. In this case, after a pattern is formed by using the photosensitive resin composition, the polyamic acid is subjected to a polyimidization to produce a coverlay film, and a high temperature of about 350° C. is required for the polyamic acid previously known in the art to become the polyimide. This is undesirable because polyimidazation through a hot thermal curing can cause oxidization and deterioration of a copper circuit that is vulnerable to heat. Moreover, a photosensitive circuit protection film such as the coverlay film should be able to fill a space between fine patterns of the circuit, but the polyimide has difficulty in realizing such a filling function. Also, since these type of photosensitive resin compositions based on polyimide have difficulties in a developing process with an alkaline aqueous solution, they are still disadvantageous in light of a working stability or the environment.